Various techniques have been known in the field to split an optical beam coming from a VCSEL mounted on a reference plane in order to monitor statuses of the VCSEL through the split beam. Japanese Patent Applications laid open Nos. of 2012-194372A and 2007-171427A, and a United States Patent Application Publication US2009/0154877 A1 have disclosed such techniques.
However, conventional techniques require complicated structure in the shell covering the VCSEL to guide the split optical beam toward a monitoring photodiode (hereafter denoted as PD) which is mounted also on the reference plane. Simple and reliable means have been requested for a long time.